In the field of guided weapons there are primarily two possible aerodynamically controlled airframes, namely, a rolling airframe and a roll stabilized airframe. These two schemes cover most of the missiles and projectiles with aerodynamic controls.
A missile or projectile with a rolling airframe has an airframe that is free to roll or its rolling motion is controlled by a device (such as a rolleron) to keep the roll rate at a certain value. Aerodynamic controlled deflections can then be coordinated with the roll position, which is calculated by roll-resolvers using roll gyros. A typical example is the Sidewinder missile, which uses four steering canards.
Another embodiment of the rolling airframe uses only one pair of aerodynamic control surfaces and deflects them in a proper position to satisfy the guidance and control vector demand.
General Dynamics pioneered several new design features to create the Redeye missile, which was the first rolling airframe missile (RAM). Unlike conventional roll stabilized missiles which are steered in two axes, pitch and yaw, by two (pitch, yaw) control channels, a RAM uses a single control channel which is “phased” to introduce pitch and yaw commands subject to the missile's instantaneous orientation (roll angle) in roll. In this fashion a single pair of control surfaces can do the work of two pairs of control surfaces, reducing weight and space requirements with some penalty in maneuver performance. General Dynamics applied further new technology to the Redeye missile by designing all of the guidance and control electronics with solid state transistor and integrated circuit technology, a first in tactical missiles. Another major weight saving measure was the use of electrical control actuators to displace bulkier conventional hydraulics. Internal wiring harnesses in the missile were replaced with lighter, flexible, flat printed wiring harnesses.
Two schemes of control by a single pair of deflecting canards have been used in RAM missiles. In one of the schemes, the canards generate the lift forces by deflecting the canards by a certain angle by an actuator according to the roll position and the lift required to generate the lateral acceleration to change the trajectory angle. In the other scheme, referred to as “Dithering Canards,” the canards, once deployed, vibrate or dither at some frequency in the rolling airframe to create the appropriate lateral force to steer the missile or projectile.
Dithering canards are simpler than deflectable canards with specific angles of deflection because it is not necessary to have a complex servomechanism to deflect them. However, the dithering canard scheme needs to be packed and then deployed after launch, which usually makes the mechanical design complex.
Seeking simplicity and low cost solutions to be used in the control of guided mortar projectiles, General Dynamics found a solution with the so-called roll controlled fixed canard (RCFC) system, as set forth in U.S. Pat. No. 7,354,017 to Morris et al. The RCFC system is an integrated fuze and guidance- and flight-control system that uses global positioning system (GPS) and/or inertial navigational system (INS) navigation and that is installed by replacing current fuze hardware in existing mortars or other projectiles. A typical projectile having the RCFC system comprises:                (a) a nose section with a guidance package, a set of spinning strakes, and a set of two fixed deflected canards;        (b) a brake unit section, with a brake system (friction or magneto rheological fluid), to modulate the spin of the guidance section with the projectile body and stabilizing fins; and        (c) a projectile body with multiple canted fins.The projectile is designed to couple and decouple the two sections (nose and main) that can rotate in different directions with variable spin rates, or rotate as a single body, dependent upon the braking force. If the rotation rate is close to zero in the reference frame, the fixed deflected canards will trim the projectile and generate lateral normal force, which will steer the projectile in the desired or demand vector requested by the guidance system (for example, GPS or INS). However, this system is quite complex and not practical for many projectiles.        
Another concept to create trajectory correction to artillery projectiles is disclosed in PCT Published Application No. WO 2008/143707 to Pritash. Trajectory errors can be corrected in two ways: Assuming an overshoot, a deployable set of brake fins or disks is used to correct the range errors. This assumes that the target is at a range shorter than at which the weapon is aimed, because it only can waste kinetic energy by braking the projectile by the use of aerodynamic brakes.
A deflection correction is based on the fact that a very fast spinning projectile will divert (drift) to one side depending of the roll motion direction, and therefore changing the roll rate changes the amount of the lateral drift. The spin correction fins of Pristash do exactly this by extending or retracting spin fins which are at fixed incidence but in opposite directions. The spin rate, and hence the deflection, is controlled. However, the gun or weapon must be aimed in a specific direction prior to shooting so that by changing the spin rate and braking the velocity over the trajectory, the desired target can be hit.
Similar to the control system discussed above, this system is much more complex than is needed for slow rolling projectiles.